Gene deletion studies have demonstrated the importance of IGF-I and II (IGFs), particularly during fetal life when local IGFs production predominates. After birth, the liver becomes the most important site of IGFs synthesis, resulting in the development of a substantial plasma reservoir. This reservoir is dependent on the postnatal production of the acid labile subunit (ALS), a protein that recruits IGFs and IGF Binding Protein-3 in long-lived ternary complexes. The significance of this reservoir has been uncertain until we showed that ALS and the plasma IGF-I reservoir are required for early postnatal growth and bone development. We now will extend these studies to normal and diseased states of later postnatal life. This is relevant to malnutrition and catabolic illnesses in which decreased plasma IGF-I is associated with erosion of lean mass. Despite this association, IGF-I-based therapies have had limited success, reflecting the need for their incorporation into ternary complexes for effectiveness. Three specific aims wilt be pursued to address the role of ALS and the circulating IGFs reservoir during diseased states. AIM A: IGF-I is a potent positive regulator of skeletal muscle mass. Null ALS mice will be subjected to challenges known to induce changes in plasma IGF-I and to alter the mass of skeletal muscles (i.e., sudden increase in GH, nutritional deficiency or sepsis). AIM B: Humans have 3 times as much plasma IGF-II than IGF-I. In contrast, mice have little IGF-II and null ALS mice have normal carbohydrate homeostasis. To determine the role of ALS in containing the metabolic effects of IGF-II, we will study null ALS mice over-expressing human IGF-II. AIM C: GH stimulates ALS synthesis by increasing transcription. In vitro, this effect is conveyed by STAT5, but the importance of this mechanism remains to be established in vivo. Using null STAT5 mice and liver cells, we will evaluate the contribution of direct and indirect mechanisms mediating the effects of GH on ALS synthesis. Studying the GH-regulation of ALS transcription will provide clues to mechanisms responsible for development of hepatic GH resistance during catabolic diseases. Overall, these studies will significantly advance our understanding of the roles played by ALS and the circulating IGF reservoir in diseases of postnatal life.